1. Field of the Invention
The present invention relates to a resin gear forming a power transmission device for various machines, such as printers, copying machines, video tape recorders and automotive parts. The present invention also relates to a rotational power transmission member of a resin, such as a resin gear, a resin pulley or a resin sprocket.
2. Description of the Prior Art
Conventional power transmission devices for machines, such as copying machines and automotive parts use resin gears in order to reduce the price, weight and operation noises of parts. Typically, such a resin gear is formed so as to have a predetermined shape by injection molding. The shape of the resin gear is devised so that the resin gear has a desired precision of tooth profile and a desired strength in accordance with intended purpose.
FIGS. 16 through 18 show such a conventional resin gear 30. In the resin gear 30 shown in these figures, a rim 31 is connected to a hub 32 by a web 33. On the side of one end of the hub 34, a key receiving portion 34 is formed. The key receiving portion 34 is designed to receive therein and engage a key 36 of a driving shaft 35 to rotate with the driving shaft 35 (see, e.g., Japanese Patent Laid-Open No. 2001-289311).
In the conventional resin gear 30 shown in FIGS. 16 through 18, the bottom wall 37 of the key receiving portion 34 and the web 37, which has a thickness substantially equal to the thickness of the bottom wall 37, are substantially arranged at the same position in width directions (in right and left directions in FIG. 17). Thus, stress (bending stress and/or shearing stress), which is caused by force applied from the key 36 during power transmission, is applied so as to concentrate on corner portions 40 which are defined by the bottom wall 37 and side walls 38 of the key receiving portion 34, so that there is the possibility that the key receiving portion 34 may be broken. In such a case, if the thickness of the web 33 is increased so that the web 33 receives stress applied on the key receiving portion 34, it is possible to prevent the key receiving portion 34 from being broken. However, in the resin gear 30, it is required to suppress the deterioration of the precision of tooth profile due to shrinkage or the like after injection molding to maintain a desired precision of tooth profile, so that the thickness of the web 33 is limited in order to maintain the precision of tooth profile. As a result, it is difficult to sufficiently ensure the strength of the key receiving portion 34 of the conventional resin gear 30.